Marathon bombing suspect Tamerlan Tsarnaev was alive and struggling with Watertown police early Friday morning, when his younger brother and alleged co-conspirator drove over him in a stolen SUV, dragging him on the pavement and apparently inflicting the fatal injuries that killed him, said Watertown Police Chief Ed Deveau in a Globe interview.

His brother, Dzhokar Tsarnaev, 19, is in Beth Israel Deaconess Medical Center in serious condition. He was captured Friday night, after hiding for hours in a boat stored in a Watertown backyard. Authorities believe the brothers planted bombs made from pressure cookers at the finish line of the Boston Marathon on Monday, killing three and wounding more than 170.

The chief described the chain of events that led to Tamerlan Tsarnaev’s death, beginning late Thursday, after the brothers allegedly carjacked a man in Cambridge. The two brothers let the driver out unharmed, but the driver’s cellphone was still in his car, said Deveau. Police “pinged” the phone to determine where it was, he said, which alerted police that the suspects were in Watertown.

A local officer spotted the brothers driving in two cars, a Honda sedan and the stolen Mercedes SUV, said Deveau. The brothers stopped, jumped out and started firing on the officer, while more police rushed to the scene, he said.

“Quickly we had six Watertown police officers and two bad guys in a gunfight,” said Deveau. At least 200 shots were fired; maybe as many as 300, he said.

The shots, around 12:50 a.m., woke resident Jennings Aske, at 66 Laurel Ave. When he looked out his window he saw a green Honda Civic sedan stopped in the street, with its lights on and the driver’s side windows blown out. A husky man – apparently Tamerlan Tsarnaev—stood nearby firing a gun.

“I saw him standing there shooting at police,” Aske said. “When he fired, there was a little flash of light. I could hear the gun firing. It was terrifying.”

A police SUV came down Laurel Street, hearing west toward Dexter Ave., crashing into some trash cans on the sidewalk and into a parked car. Aske said he then saw a black Mercedes SUV drive up, also heading west toward Dexter Avenue. Then he heard two or three loud explosions, one of them large enough to shake the house. In an instant, the street “was crawling with police,” he said.

Deveau said the Tsarnaev brothers hurled something at the officers – apparently a pressure cooker bomb—and there was a tremendous explosion. Police later found the lid to a pressure cooker. “We believe it was an exact duplicate of the Boston Marathon bombs,” he said. The suspects also threw five “crude grenades” at officers; three of which exploded, he said.

One of his officers put his cruiser into gear and jumped out of it, letting it roll at the suspects to draw fire, he said. The suspects peppered the car with bullets.

After several minutes, the elder brother, Tamerlan, walked toward the officers, firing his gun until he appeared to run out of bullets, Deveau said. Officers tackled him and were trying to get handcuffs on him, when the stolen SUV came roaring at them, the younger brother at the wheel. The officers scattered and the SUV plowed over Tamerlan Tsarnaev, who was dragged briefly under the car, he said.

Dzhokar Tsarnaev abandoned the SUV almost immediately on a nearby street and fled on foot, triggering an all-day manhunt.

Meanwhile, officers at the scene treated MBTA Transit Police Officer Richard H. Donohue Jr., who had arrived on the scene and was wounded in the gunfight.

Aske continued to watch for several hours, while police searched the abandoned Civic with a robot, apparently looking for explosives.

The state medical examiner has not released an autopsy report detailing cause of death.